Diffuse Colour not showing in compositing render layers
Your material simply has no diffuse component, which is why diffuse direct, indirect and color passes are completely black. This happens because the material is partly metallic and uses subsurface scattering.

The way the light passes are combined to produce the final image is documented in the manual.


The Blender 2.80 Manual by the Blender Documentation Team is licensed under CC-BY-SA v4.0.